1. Field of the Invention
The present invention generally relates to optical fiber communications systems, and more particularly to a transmission line switching system for switching from an optical transmission line in an active system to an optical transmission line in a protection system when a fault has occurred in the above optical transmission line in the active system.
2. Description of the Related Art
Recently, optical fiber communications systems capable of transferring a large quantity of data by means of an optical fiber have been proposed. A fault which has occurred in an optical fiber will cause great damage to users. Hence, it is very important to ensure reliability of an optical transmission system including optical fibers.
FIG. 1 shows a conventional optical communications system of a synchronous type, which includes a protection-system transmission line L-P and an active-system transmission line L-S1, which is now used for communications. In actuality, the communications system includes a plurality of active-system transmission lines. The transmission lines L-P and L-S1 are respectively made of optical fibers. Two transmitter/receiver devices 1-P in the protection system are connected to the respective ends of the protection-system line 1-P. Similarly, two transmitter/receiver devices 1-1 in the active system are connected to the respective ends of the active-system line. Each of the transmitter/receiver devices 1-P and 1-1 is made up of a low-bit-rate interface unit 2, a multiplexer/demultiplexer unit 3, and a high-bit-rate interface unit 4. A switching controller EX, which is connected to, for example, terminals (not shown), is connected in common to the low-bit-rate interface units 2 of the transmitter/receiver devices 1-P and 1-1. FIG. 1 shows two switching controllers EX. The interface unit 2 has the function of converting a light signal into an electric signal and vice versa.
During normal operation, the transmitter/receiver devices 1-1 in the active system communicate with each other, and the transmitter/receiver devices 1-P in the protection system communicate with each other. If a fault has occurred in the transmission line L-S1 in the active system, a monitor device (not shown) for monitoring the status of the transmission lines L-P and L-S1 detects the above fault, and controls the switching controllers EX so that low-bit-rate data is sent to and received from the transmitter/receiver devices 1-P in the protection system.
FIG. 2 shows another conventional optical communications system. The optical communications system shown in FIG. 2 does not include the switching controllers EX shown in FIG. 1. The high-bit-rate interface unit 4 of each of the transmitter/receiver units 1-P and 1-1 is composed of a first high-bit-rate interface unit 4S in the active system and a second high-bit-rate interface unit 4P in the protection system.
During the normal operation, the high-bit-rate interface units 4S in the active system communicate with each other, and the high-bit-rate interface units 4S in the protection system communicate with each other. If a fault has occurred in the transmission line L-S1 in the active system, a monitor device (not shown) controls the transmitter/receiver units 1-P and 1-1 so that the second high-bit-rate interface units 4P in the protection system start to operate. Then, the high-bit-rate interface units 4P in the protection and active systems are connected to each other, as shown in FIG. 2. In this manner, the transmission line L-S1 in which a fault has occurred is replaced by the transmission system L-P in the protection system, and data transfer continues.
However, the conventional system shown in FIG. 1 has a disadvantage in that it needs the switching controllers EX and hence the system has a large size. In addition, there are various interface types, such as an electric type and an optical type, and hence it is necessary to provide the low-bit-rate interface units for the respective interface types.
The conventional system shown in FIG. 2 has the following disadvantages. In general, synchronizing signal sources (not shown) provided for the multiplexer/demultiplexer units 3 with timing signals may be separately provided in the protection system and the active system, and normally do not operate with identical timings. Strictly speaking, the frequencies of the synchronizing signals may be slightly different from each other. When a fault has occurred in the transmission line in the active system, a connection is made between the high-bit-rate interface units 4P respectively provided in the protection system and the active system. The synchronizing signal sources in the protection system and the active system operate independently of each other, and hence it is not possible to directly send data to the transmission line 1-P in the protection system. If data is directly sent to the transmission line 1-P, an instantaneous break will occur. As a result, data from the active system is supplied to the multiplexer/demultiplexer unit 3 in which the data is synchronized with the synchronizing signal in the protection system and overhead bits are multiplexed with the data. The above operation of the multiplexer/demultiplexer unit 3 is very complex.